Tapping on “export as image” is a noop. Other options work. Arc 3.17.1, iOS 18.5.
Wanted to do that right now after an event, and 
Ah that old bug. It’s probably because the timeline you’re trying to export results in a too-large image file, so iOS just quietly aborts without telling Arc that it’s done that. So Arc happily goes on thinking “all succeeded!” when it really hasn’t.
For years now I’ve had the plan to rebuild the “Export as image” feature to try to break it up into smaller parts, then stitch the images together afterwards. But it’ll be fiddly, and I’ve never found the time.
Hopefully it’ll either Just Work in Arc Editor app, or it’ll be much easier to do the workaround.